The present invention relates to a method for making lithographic printing plates from presensitized plates for making the same (hereunder referred to as "PS plate(s)") and more particularly to a method for making lithographic printing plates by imagewise exposing a PS plate, whose substrate is made of a metal, to light, developing the exposed PS plate and then subjecting it to burning-in treatment; and a solution for the burning-in pretreatment.
Lithographic printing technique makes the best use of the properties of water and an oil such that they are essentially incompatible with one another. The printing surface thereof comprises areas which receive water and repel an oil ink and those which repel water and receive an oil ink, the former being non-image areas and the latter being image areas. Therefore, light-sensitive compositions used to obtain lithographic printing plates should have properties such that after the formation of images, they repel water while they receive an oil ink.
The light-sensitive compositions used to form lithographic printing plates are divided into two groups, i.e., positive-working and negative-working type ones. Widely used positive-working light-sensitive compositions are composed of o-quinonediazide compounds.
Positive-working PS plates in general comprise a substrate of a metal or a plastic to which an o-quinonediazide compound alone or combined with an alkali-soluble resin such as a novolak type phenol resin or a cresol resin is applied. When such a light-sensitive layer is exposed to an actinic radiation through a positive transparency, the o-quinonediazide compound existing at the exposed portions thereof is decomposed and made alkali-soluble. Therefore, such PS plates can easily be developed with an alkaline aqueous solution to provide positive images. In this case, if a substrate having a hydrophilic surface is employed, a part of the hydrophilic surface from which the light-sensitive layer is removed by an alkaline aqueous solution is revealed to form a non-image area and the exposed portions receive water and repel ink. On the contrary, the remaining portions serving as image portions are oleophilic and thus receive ink.
On the other hand, most of the negative working light-sensitive compositions comprise diazonium salts, azide compounds or photopolymerizable compounds. Such a negative working light-sensitive composition which is used alone or in combination with such an additive as a resin binder is applied to the surface of a substrate. If a substrate having hydrophilic surfaces is utilized to form a light-sensitive composition, the light-sensitive compositions in unexposed portions of the layer formed therefrom are removed with a developer to expose the hydrophilic surface of the substrate, which receive water and repel ink. On the contrary, the remaining portions serving as images after development are oleophilic and receive ink.
If the lithographic printing plate thus obtained is set up on an offset printing press to perform printing operation, beautiful printed matters are provided. The lithographic printing plate produced from such a PS plate can provide several tens of thousands of printed matters if materials for substrate and the composition of the light-sensitive layer applied thereon are appropriately selected. In particular, beautiful printed matters as much as a hundred thousands can be obtained if an aluminum plate which has been grained and then anodized is adopted as a substrate therefor.
Nevertheless, it is required to develop a further improved printing plate capable of providing much more greater number of printed matters. One effective method complying with such a requirement comprises exposing a PS plate whose substrate is made of a metal such as aluminum or zinc to light, developing it and then heating it at an elevated temperature (so-called burning-in treatment) to strengthen the image portions thereof. The burning-in treatment makes it possible to increase the number of printed matters obtained from only one lithographic printing plate as much as several times greater than that achieved by printing plates which are not subjected to a such burning-in treatment.
Moreover, if printing is carried out using a special ink such as ultraviolet-curable ink or low temperature drying ink containing a variety of components which possibly causes dissolution of the images on the printing plate, the image portions are extremely dissolved out when the printing plate is not subjected to burning-in treatment and the number of printed matters acceptable is substantially lowered compared with that obtained when a common ink is used. On the contrary, if the printing plate is subjected to burning-in treatment, the resistance of the image areas to solvent is substantially enhanced and, therefore, sufficient number of printed matters can be obtained even if the aforesaid special ink is used.
However, the hydrophilicity of the non-image areas (i.e., portions at which the hydrophilic surface of the substrate is exposed through development) of the printing plate is impaired due to the burning-in treatment and is converted to ink-receptive one. This results in background contamination on printed matters. When the heating is performed so that such background contamination can be prevented, satisfactory burning-in effect cannot be achieved and hence enhancement of images cannot also be achieved. Therefore, it is required to necessarily carry out counter-etching of the surface of the plate for preventing the contamination of non-image areas resulting from such burning-in treatment. There have been proposed a variety of such counter-etching of the surface performed before and/or after the burning-in treatment.
For instance, contaminations caused due to the burning-in treatment are cleaned thereafter and a processing solution for recovering hydrophilicity of the non-image areas, such an aqueous solution of a fluoride as those containing hydrofluoric acid, borofluoric acid, hydrosilicofluoric acid is applied to the non-image areas. However, these fluorides are toxic substances or violent poisons and, therefore, a problem concerning environmental pollution arises.
Moreover, the counter-etching causes corrosion of the metal surface of the substrate, the surface is in turn easily scratched and shows low wear resistance. As a result, the surface loses its property for receiving water on the non-image areas during printing operation (i.e., water retention), background contamination is caused and thus the plate cannot stand further printing.
In addition, Japanese Patent Un-examined Publication (hereunder referred to as "J.P. KOKAI") No. 51-34001 discloses a method for preventing the lowering of the hydrophilicity of non-image areas prior to such a burning-in treatment, the method comprising treating a plate with an aqueous solution of a salt of organic sulfonic acid such as sodium alkylnaphthalenesulfonate, sodium alkyl diphenyl ether sulfonate or lithium nitrate, prior to the burning-in treatment. Among these, the aqueous solution of a salt of an organic sulfonic acid such as sodium alkylnaphthalenesulfonate or sodium alkyl diphenyl ether sulfonate is highly foamy and thus is not applicable to a spray-circulation type processing apparatus presently used widely. This makes it difficult to automate the plate making operations. Moreover, if the plate is treated with an aqueous solution containing lithium nitrate and then subjected to burning-in treatment, it is not possible to sufficiently prevent occurrence of background contamination.
On the other hand, Japanese Patent Publication for Opposition Purpose (hereunder referred to as "J.P. KOKOKU") No. 55-28062 (U.S. Pat. No. 4,063,507) discloses a method for treating the plate with an aqueous solution containing boric acid capable of causing sublimation and a salt thereof and then subjecting it to burning-in treratment in the presence of such compounds. However, this method cannot completely prevent background contamination. In particular, background contamination is likely caused depending on conditions under which subsequent processings such as water washing and gumming are conducted. For instance, insufficient washing with water and the use of desensitizing gumming solution having low hydrophilization power such as a dextrin type gumming solution easily cause background contamination.
In addition, J.P. KOKAI No. 52-6205 (Canadian Patent No. 1,084,758) discloses a method for treating a plate, prior to a burning-in treatment, with an aqueous solution of a water-soluble organic substance such as gum arabic, cellulose ether or polyacrylic acid and/or an aqueous solution of a water-soluble inorganic salt such as a borate, a phosphate, a sulfate or a halide of alkali metals or alkaline earth metals. However, the method cannot necessarily prevent background contamination completely and in particular if an aqueous solution of a water-soluble polymer such as gum arabic or polyacrylic acid is used, ink receptivity of the resultant image areas during printing is often reduced, in other words the image areas cause image blinding.
J.P. KOKAI No. 57-52057 (U.S. Pat. No. 4,355,096) discloses a method for treating a plate, prior to a burning-in treatment, with an aqueous solution containing an amine having carboxyl group(s) such as ethylenediamine-tetraacetic acid or hydroxyalkyl ethylenediamine-triacetic acid or a salt thereof. However, background contamination inhibitory effect attained by this method is likewise insufficient.
Referring to contamination of half-dot image portions, particularly in the shadow tones (hereunder referred to as "ink spreading"), it is found that if a plate is treated with an aqueous solution containing a surfactant such as a salt of an organic sulfonic acid prior to a burning-in treatment, contamination is not observed while if it is treated with an aqueous solution free of such a surfactant, such portions are likely to cause contamination. As already discussed above, however, the aqueous solution containing a surfactant is very foamy and thus is not applicable to a spray-circulation type processing apparatus presently used widely. This makes it difficult to automate plate making operations.